The invention relates generally to frequency diplexers for separating coincident microwave signals of different frequency, and in particular, to a double square loop frequency selective surface which provides polarization independent separation of microwave signals in two closely adjacent frequency bands.
A diplexer is, in general terms, any device utilized to separate coincident signals of different frequency. In microwave communication systems a diplexer can be provided in the form of a frequency selective surface or a dichroic surface. The frequency selective surface typically comprises a grid or an array of conductor elements formed on a dielectric substrate. The geometric configuration of the conductor elements and their connection or non-connection produces a surface which exhibits transmission resonance to incident microwave signals within a predetermined frequency band. Typically, the frequency selective surface is used in conjunction with a primary paraboloidal reflector. The frequency selective surface is positioned between the primary reflector and its focal point. Signals of the resonant frequency are reflected by the frequency selective surface while other signals are transmitted thereby. This effectively separates the signals for further processing.
Examples of such prior art frequency selective surfaces can be found in U.S. Pat. No. 4,125,841 to Munk which utilizes multiple aligned screens to provide a frequency selective surface having a desired angular transmission characteristic. Such multiple layer screens have the disadvantage of additional weight and bulk required by the multiple layers and increased cost as a result of precise alignment requirements. Single layer frequency selective surfaces are disclosed in U.S. Pat. No. 4,785,310 to Rosen and U.S. Pat. No. 4,814,785 to Te-Kao Wu, both of which are assigned to assignee of the present invention. These patents disclose frequency selective surfaces which overcome the need for multiple screens and their attendant complexity.
Frequency selective surfaces comprised of an array of double square loops are disclosed in the patent entitled "Double-Square Frequency Selective Surfaces and their Equivalent Circuit," by Langly and Parker, Electronics Letters, Aug. 18, 1983, Vol. 19, No. 17. In this paper, a frequency selector surface capable of separating signals having a frequency ratio of as low as about 1.4 is derived with the frequencies of the reflected and transmitted signals being a function of the perimetral dimensions of the inner and outer square loops. The paper discloses a mathematical model and equivalent circuit for a double square loop frequency selective surface. The resonant and transmitted frequencies of the surface are described as a function of the perimetral dimensions of the square loop conductor elements.
In many microwave communication systems, it is necessary to separate microwave signals in closely adjacent bands, the Ka and Ku transmit and receive bands, for example. In such applications, the ration of the two frequency bands is in the order of 1.15. There therefore exists a need for a frequency selective surface that can be provided as a single layer surface which exhibits a sharp cut off transmission response allowing separation of frequency bands having a ration of about 1.15 and which has a symmetrical configuration making the surface polarization independent.
It is therefore an objective of the invention to provide an improved frequency selective surface. Another objective of the invention is to provide a frequency selective surface comprised of a symmetrical array of conductor elements that is polarization independent. Yet another objective of the invention is to provide a frequency selective surface which exhibits a very sharp frequency band pass characteristic enabling separation of closely adjacent frequency band signals. Another objective of the invention is to provide a frequency selective surface that exhibits sharp frequency cut off characteristics with a surface comprised of a single layer of conductor elements. Still another objective of the invention is to provide a frequency selective surface which can separate frequency bands having a frequency ratio of 1.15.